Due to many advantages, CZTS is a promising semiconductor compound material for solar cell thin film applications. First, the band gap EG of the material is 1.4 to 1.5 eV and is thus near the optimal value for photovoltaic application. This material also has a high optical absorption coefficient of above 104 cm−1. In addition, all the constituents of the material are non-toxic elements and are abundant on earth (H. Katagiri et al., “Development of Thin Film Solar Cell Based on Cu2ZnSnS4 Thin Films”, Solar Energy Mat. & Solar Cells, 65 (2001), 141-148; H. Katagiri, “Cu2ZnSnS4 Thin Film Solar Cells”, Thin Solid Films, 480-481 (2005), 426-432; K. Moriya et al., “Characterization of Cu2ZnSnS4 Thin Films Prepared by Photo-Chemical Deposition”, Jap. J. Appl. Phys., 44 (1B) (2005), 715-717).
Solar cells are devices that have the characteristics to convert the energy of light into electric energy. Several systems have been developed to maximize the amount of light intercepted and converted to electricity, such as optical concentrators or multi-junctions cells, said cells consisting of junctions with different band gaps (highest band gap on top). Part of the non-absorbed light in the uppermost junction will be absorbed by the second junction lying there beneath, and further non-absorbed light will be absorbed by further junctions lying beneath said second junction and so forth.
A conventional thin film solar cell is composed of a stacking of thin layers on a flat substrate, said thin layers forming the junction. In order to intercept as much sunlight as possible, the visible surface of the junction is maximized, in particular by using a front contact being formed from a thin grid.
CZST thin films are p-type and can serve as an absorber layer in thin film solar cells (K. Moriya at al., ibid.). Various processes for the deposition of CZTS thin films are described in the literature. In 1988 K. Ito at al. formed quaternary stannite-type semiconductor films of CZTS with (112) orientation on heated glass substrate using atom beam sputtering. The sputtering target was made from a synthesized powder of the quarternary compound. Pure argon was used as a sputtering gas (K. Ito et al., “Electrical and Optical Properties of Stannite-Type Quarternary Semiconductor Thin Films”, Jap. J. Appl. Phys., 27 (11) (1988), 2094-2097).
Later they continued their studies by using a spray hydrolysis technique for synthesis of the CZTS thin films (N. Nakayama et al., “Sprayed Films of Stannite Cu2ZnSnS4”, Appl. Surf. Sci., 92 (1996), 171-175.
J. Madarász at al. applied a spray pyrolysis deposition technique in the same way (J. Madarász at al., “Thermal Decomposition of Thiourea Complexes of Cu(I), Zn(II) and Sn(II) Chlorides as Precursors for the Spray Pyrolysis Deposition of Sulfide Thin Films”, Solid State Ionics, 141-142 (2001), 439-446).
The layers produced by decomposition of thiourea from the sprayed electrolyte parallel to a co-deposition of metal chlorides produced almost stoichiometric compositions. In both working groups an annealing step after the deposition was necessary.
All these studies were focused to collect basic physical data, such as band gap energy and absorption coefficient. These studies confirmed that kesterite (Cu2ZnSnS4) is a hopeful candidate to be used as a thin film solar cell material. This material is a p-type semiconductor which is has a tetragonal structure derived from the ZnS sphalerite (or zinkblende). The documents mentioned, however, do not disclose any solar cell conversion efficiencies and the research groups mentioned did not report such data until today.
In 1997 H. Katagiri at al. published an alternative CZTS synthesis. In the first step a metal sandwich layer (Cu/Zn/Sn) was deposited by PVD technique while in a following step vapor-phase sulfurization was performed. The stacked precursors on the substrate were formed by depositing ZnS first, next Sn was deposited and last Cu was deposited by sequential evaporation. (H. Katagiri at al., “Preparation and Evaluation of Cu2ZnSnS4 Thin Films by Sulfurization”, Solar Energy Mat. & Solar Cells, 49 (1997) 407-414).
In 2003 J.-S. Seal at al. have reported applying a PVD process (rf magnetron sputtering) for CZTS precursor deposition (J.-S.Seol at al., “Electrical and Optical Properties of Cu2ZnSnS4 Thin Films Prepared by RF Magnetron Sputtering Process”, Solar Energy Mat. & Solar Cells, 75 (2003), 155-162).
H. Katagiri reported in 1997 data of a CZTS based solar cell to show an efficiency of 0.66% (H. Katagiri et al., ibid. (1997)). The reported efficiency improved to 2.62% in 2001 (H. Katagiri at al., ibid. (2001)). This reported efficiency also exceeded the efficiency reported by Th. M. Friedlmeier at al. in 1997 which was referred to be 2.3% (Th. M. Friedlmeier et al., “Growth and Characterization of Cu2ZnSnS4 and Cu2ZnSnSe4 thin Films for Photovoltaic Applications”, Inst. Phys. Conf. Ser. No, 152: Sec. B: Thin Film Growth and Characterization (1998), 345-348).
Recently pure metal sputtering targets were replaced by metal sulfide ones to avoid expansion effects during the sulfurization step. From the scientific literature H. Katagiri et al. are estimated to be the leading research group which was confirmed in 2005 (H. Katagiri, ibid. (2005)) by a notice by reference to H. Katagiri at al., “Solar Cell Without Environmental Pollution by Using CZTS Thin Films”, Proceedings of the 3rd World Conference on Photovoltaic Energy Conversion, Osaka, 2874 (2003), that the so far highest reported efficiency value of 5.45% would have been reached in 2003.
Coatings of an alloy comprising copper, zinc and tin, having different atomic ratios, and which are produced by electrodeposition, have an outstanding corrosion resistance. Hence, these systems have gained attention as a replacement for nickel. However, most of the commercially available plating solutions contain free cyanide to maintain high stability of the plating composition used and a constant metal composition:
In U.S. Pat. No. 2,435,967 an electroplating bath for depositing a bright silvery alloy plate is disclosed, which is composed of 50% to 75% copper, 15% to 30% tin and 5% to 20% zinc. The bath comprises, in combination, an aqueous electrolyte composed of free cyanide, copper, tin, zinc, alkali metal hydroxide, an alkali metal carbonate and an anti-pitting and brightening agent composed of a betaine having at least one non-cyclic hydrocarbon radical which contains from 10 to 20 carbon atoms.
U.S. Pat. No. 2,530,967 discloses a method of electrodepositing bright silvery corrosion-resisting coatings of a ternary alloy composed of from 50% to 75% copper, 15% to 30% tin and 5% to 20% zinc. An aqueous solution for electroplating this alloy is used, this solution comprising free cyanide, copper, tin, zinc, alkali metal hydroxide and alkali metal carbonate.
Further, general literature information on basic bath compositions and on monitoring is summarized in Metals Handbook, American Society for Metals, 9th ed. (1993), USA. Reference to bronze plating and ternary alloys of copper and tin that are alloyed with other metals can be plated, but control of the plating process is considered so difficult that they have found very limited use. In general, plating compositions are used which contain excess free cyanide.
In 1971 G. F. Jacky examined electroplating of copper-tin-zinc alloys. Estimated contents of the deposited layers ranged from 55% to 60% copper, 20% to 30% tin and 10% to 20% zinc. An alkaline cyanide aqueous plating bath was used. (G. F. Jacky, “Electroplating a Copper-Tin-Zinc Alloy”, Plating 58 (1971), 883-887). The high-performance of this coating system was outlined without giving any detailed information about long term stability or layer composition adjustment.
In 1985, the commercial system Optalloy® (trade mark of Collini-Flühmann, CH) was claimed to be more stable than competing bath systems. J. Cl. Puippe used this commercial bath to study influence of pulsed currents on alloy plating. Besides enhanced reproducibility and improved surface appearance, even adjustment of coating composition could be achieved while using pulse plating (J. Cl. Puippe, “Optalloy—eine elektrolytisch hergestellte Cu—Zn—Sn-Legierung”, Galvanotechnik, 76 (1985), 536-441).
In the same year further reports about the commercial Cu—Zn—Sn application Optalloy® confirmed the predicted properties of this electrolyte system (R. Jurczok et at, “Anwendungstechnische Eigenschaften von galvanischen Kupfer-Zinn-Zink-Schichten in der Elektroindustrie”, Metalloberfläche, 39 (1985), 201-203).
L. Picincu et al published an investigation about the deposition kinetics of the ternary alloy process in 2001 by using the cyanide containing commercial plating bath Sucoplate® (trade mark of Huber+Suhner AG, CH) (L. Picincu et al., “Electrochemistry of the SUCOPLATE® Electroplating Bath for the Deposition of Cu—Zn—Sn alloy; Part II: Influence of the Concentration of Bath Components”, J. Appl. Electrochem., 31 (2001), 395-402). Alloy compositions were obtained ranging from 47% to 51% copper, 8% to 12% zinc and 38% to 43% tin. The electrolyte compositions used to plate these alloys are reported to contain cyanide.
All so far published copper-zinc-tin bath systems would require a fundamental optimization to achieve the metal ratios necessary for a CZTS solar cell precursor (Cu: 50 at.-%, Zn: 25 at.-%, Sn: 25 at.-%). Due to the very high toxicity of cyanide it would be more favourable to develop a non-cyanide plating system. This demand is not served from any known electrolyte.
On a first glance, scientific literature seems to offer several binary electrolytes (Cu/Sn; Cu/Zn; Sn/Zn). However, looking closer most of them cannot fulfil the demand of Cu2ZnSn stoichiometry.
There are just some patents on electrodeposition processes for CuZnSn without using cyanide as a complexing agent. It is known that pyrophosphate based electrolytes offer a high stability and good plating properties. Nevertheless, the low metal contents in the electrolytes described as well as the resulting alloy composition do not fulfil the demand of Cu2ZnSn stoichiometry.
JP 63-206494 A relates to bright copper-tin-zinc alloy electroplating baths free of cyano compounds. These baths comprise salts of Cu, Zn and Sn, alkali metal salts of pyrophosphoric acid and/or polyphosphoric acid, one or more hydroxycarboxilic acid and/or the salts thereof and one or more amino acids and/or the salts thereof. As an example an iron plate was electroplated in a bath consisting of copper pyrophoshate, zinc pyrophospate, potassium stannate, potassium pyrophosphate, sodium polyphosphate, sodium citrate and glycine at 35° C., pH 10.8 and at 0.3-5 A/dm2 cathodic current density to give a bright golden coating. However, the CuZnSn layer deposited does not exhibit the stoichiometry necessary for the use as thin film solar cells. The attempt to increase the tin and zinc content of the deposited layers causes precipitation of the electrolyte.
SU 1236010 A relates to an electrolyte for the deposition of coatings of copper-zinc-tin alloys, which comprises copper sulfate, zinc sulfate, tin sulfide and potassium pyrophosphate. The baths additionally contain polyvinylpyrrolidone. Deposition is performed at 20-40° C., 0.5-1.8 A/dm2 and pH 7-8.5. Tin content in the alloy deposited is 1-3 at.-%. These highly copper-rich deposits (72-90 at.-%) are used to produce structures of low friction for machine engineering devices. Such alloys are further taught to have low internal stress.
Further, K. Moriya et at, “Characterization of Cu2ZnSnS4, Thin Films Prepared by Photo-Chemical Deposition”, Jap. J. Appl. Phys., 44 (1B9 (2005), 515-717, disclose the preparation of CZTS thin films by photo-chemical deposition. This method starts from an aqueous solution containing copper sulfate, zinc sulfate, tin(IV) sulfate and sodium thiosulfate. The solution is flown on a soda-lime glass substrate which is illuminated with Deep-UV light of 254 nm. The films deposited are annealed in an atmosphere of N2 at 200° C., 300° C. or 400° C. The films obtained are p-type semiconductors and show photoconductivity.
The segregation of secondary phases in the quaternary system copper-zinc-tin-sulfur due to non-stoichiometry has been studied by Th. M. Friedlmeier, “Multinary Compounds and Alloys for Thin-Film Solar Cells: Cu2ZnSnS4 and Cu(In,Ga)(S,Se)2”, Fortschr.-Ber. VDI, Reihe 9, Nr. 340, VDI-Verlag, Dusseldorf (2001). These secondary phases are the ternary compound Cu2SnS3 (copper tin sulfide, CTS) and the binary sulfides sphalerite ZnS, chalcocite CuS, Cu1.8S, SnS and SnS2.
No other quaternary compounds have been reported in the kesterite system.
Cu2SnS3 has a cubic structure with lattice parameters similar to kesterite 5.445 Å, lattice mismatch: 0.37%). CIS forms a solid solution with CZTS, up to a total amount of more than 50 wt.-% of CTS in CZTS.
ZnS obviously has a structure and lattice parameters similar to kesterite, but no solid solution is reported in this case, ZnS segregates from CZTS, but the morphology of this segregation has not been studied: So, both grain boundaries precipitation and precipitation inside the grains of kesterite are possible.
Cu1.8S has a cubic structure with a lattice mismatch of 2.6% to kesterite, but the miscibility of both compounds has not been studied. Th. M. Friedlmeier presumes that copper sulfides Cu1.8S and CuS could precipitate at the grain boundaries or even inside the grains.
Tin sulfides SnS and SnS2 have an orthorhombic and hexagonal structure, respectively, with lattice constants too different from kesterite's for a solid solution to form. These phases are thus expected to segregate from kesterite.
The non-stoichiometry of elements in the kesterite would thus be accommodated as follows: excess of both elements copper and tin would result in a solid solution of Cu2SnS3; excess of copper only would result in segregation of copper sulfides CuS and Cu1.8S (with predominance of CuS when sulfurization is processed under S excess); excess of zinc would result in segregation of ZnS phase; and excess of tin only would result in precipitation of tin sulfide phases SnS and SnS2.
Owing to the complexity of the phase system on the one hand and to the difficulty to accurately analyse these phases within a thin film, the effect of the presence of secondary phases in the kesterite thin film on optical and electrical properties has not be investigated yet.
Nevertheless, copper sulfide is known to be highly conductive and may cause shunt paths through the absorber layer. The ternary compound CTS is also suspected to have the same effect, the solid solution CZTS-CTS being much more conductive than pure CZTS.